/Engine/Source/Runtime/Engine/Classes/Materials/MaterialInterface.h
Force the streaming system to disregard the normal logic for the specified duration and instead always load all mip-levels for all textures used by this material.
virtual void SetForceMipLevelsToBeResident ( bool OverrideForceMiplevelsToBeResident, bool bForceMiplevelsToBeResidentValue, float ForceDuration, int32 CinematicTextureGroups, bool bFastResponse )